Research indicates that many males above the age of 65 typically have lost about 10 dB of their overall hearing capability. This hearing loss is usually frequency-dependent, for example, those with a 10 dB overall hearing impairment have often lost about 20 dB of their hearing capability for frequencies above 2000 Hz. This hearing loss may make it difficult for persons suffering from hearing impairment to use certain audio equipment such as telephone equipment.
Thus, it is obvious that, in order to meet all the needs of those who have suffered hearing loss, it is necessary to provide telephone equipment which can amplify audio signals across the whole frequency band of human speech. Such telephone equipment must provide equalization over multiple frequency bands and selective amplification, whereby a user can tailor both high and/or low frequency amplification specifically for a particular hearing need. Several prior art methods have been used to provide this amplification. These include add-on amplified speakers which can be added to existing telephone equipment and telephone equipment which incorporates its own add-on speakers.
Most of the add-on amplified speakers sold on the market today can provide a user with adjustable amplification of audio signals by means of an add-on box, coupled in the telephone cords between the telephone base unit and the telephone handset. Generally, these units can amplify the audio signal for use by hearing impaired persons, but the units generally cannot provide sufficient amplification required by severely hearing-impaired persons or required to use the telephone equipment in a very noisy environment, such as in factories or airport facilities.
Alternatively, telephone equipment which incorporates audio amplification as an integral part of the telephone, can provide sufficient amplification for all uses, but such equipment generally does not provide equalization over multiple frequency bands or selective amplification.
An additional problem in common with both add-on and integral speaker amplification units is a phenomenon known as "howling" which arises when high amplification (e.g., greater than 10 dB) is provided by the units, causing an acoustic coupling between the handset microphone and speaker. Howling is a persistent problem in a telephone system for the hearing impaired since signal amplification of about 20 dB in a frequency band of interest is generally required by the listener. If the amplification is sufficient, the coupling will cause feedback oscillation at some resonant frequency which is determined by the sensitivity of the microphone and the characteristics of the speaker. In most microphone/speaker systems, this resonant frequency will usually occur in the frequency range between 1000 and 2500 Hz. Consequently, the feedback oscillation can be heard by the user and a notch filter cannot be used to eliminate the oscillation since the oscillation frequency range overlaps the frequency band of human speech.
Attempts have been made to determine the resonant frequency for a particular microphone and speaker combination and then to use a notch filter to remove that particular frequency. However, these attempts have failed, due to the fact that harmonics of the resonant frequency will be present and the feedback will still be amplified at those harmonics. If an attempt is made to remove the lowest harmonics, a "moving target" effect occurs, where an oscillation will occur at the frequency of the lowest harmonic which is not removed up to the point where the harmonics become audibly negligible.
Therefore, it is an object of the present invention to provide an amplification system which can amplify audio signals across the whole frequency band of human speech.
It is another object of the present invention to provide an amplification system which can provide equalization over multiple frequency bands and selective amplification, whereby a user can tailor both high and/or low frequency amplification specifically for a particular hearing need.
It is still a further object of the present invention to provide an amplification system which can produce sufficient amplification for all uses.
It is still a further object of the present invention to provide an amplification system which can reduce feedback oscillation in an amplified speaker system.